1. Technical Field
This invention relates generally to input and output scanners, and more particularly to such a scanner having a multifaceted rotating polygon that directs a beam of light through a post-facet lens system toward a surface to be scanned.
2. Background Information
A multifaceted rotating polygon can appear to wobble. That is because not all facets are exactly parallel to the axis of rotation and that bearing free play can cause the axis to tilt. So the facets appear to wobble as the polygon rotates and that condition can cause scan-to-scan spot position errors at the surface to be scanned (subsequently referred to as the photoreceptor).
In order to compensate, some early scanners included a wobble-correcting cylinder lens in a post-facet position between the polygon and the photoreceptor. Sometimes referred to as a positive pyramid error compensating cylinder lens, it helped focus a beam reflected by the facet along a desired scan line at the photoreceptor despite alignment errors in the cross-scan plane between adjacent facets. In that regard, the plane containing both the light beam and the scan line is referred to as the scan plane while a perpendicular plane containing the central position of the light beam (i.e., the position occupied by the light beam when it is directed toward the center of the scan line) is referred to as the cross-scan plane. The cylinder lens had little or no power in the scan direction so that it had essentially no effect in the scan plane, but it had power in the cross-scan direction and so sagittal field curvature was objectionable, especially as the scan angle increased with a decrease in facet-to-photoreceptor distance.
Some scanners had no optics between the facet and the photoreceptor to correct for field curvature (i.e., to flatten the field). Others simply adapted known optical designs, such as those referred to as the Cooke Triplet and the Double Gauss. But then the two-element "f-theta" lens appeared (f representing the focal length and theta the scan angle). It helped flatten the field as described in U.S. Pat. Nos. 4,108,532 (Minoura) and 4,179,183 (Tateoka and Minoura). In addition, using two elements to flatten the field left a free design parameter available for use in correcting some other design problem. It was used to compensate for scanner non-linearity.
Scanner non-linearity refers to the change in spot velocity occurring as the light beam scans across the photoreceptor. That change occurs for a constant polygon rotational rate because the spot on the photoreceptor is farther from the facet at the ends of the scan line than it is at the central position. So, with the f-theta lens configured to compensate for it, such compensation became an attribute that is still sought in post-facet lens systems.
The Minoura patents taught that linearity can be treated as distortion, a known aberration. Therefore, introducing third order barrel distortion of the proper amount cancels the third order term of spot velocity. Then, to third order accuracy, the spot velocity is constant with constant angular velocity. Hence the term "f-theta" instead of "f-tangent theta," which was the case before introduction of the f-theta lens.
Although the f-theta lens flattened the field and compensated for scanner non-linearity, compensation for wobble was left to other means. As a result, many existing scanners include a two-element f-theta lens and a wobble correcting element, for a total component count of three. In addition to the drawback of increased component count, forcing distortion onto the f-theta lens design can be a significant penalty. It can complicate the design, increase cost, and produce unwanted aberrations such as fifth order field curvature. Thus, it is desirable to have some way to simplify scanners in that respect.